IP Multicast
Stephen Li
CSC457 Computer Networks
University of Rochester
6/21/2016
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Outline
• Introduction of multicast
• Model of a Host IP Implementation
• Intra-Domain Multicast
• Inter-Domain Multicast
• Summary
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What is multicast?
• One –to- many, many –to- many data
transmission
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Why multicast?
• Videoconferencing;
LAN 1
internet
router
LAN 3
LAN 2
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Address for Multicast
• Ethernet multicast address
• First bit is set 1
• IP multicast address (Host group address)
• Class D
(from 224.0.0.0 to 239.255.255.255)
• 224.0.0.0 is not assigned to any group
• 224.0.0.1 is assigned to the permanent group of all
IP hosts
• IP multicast address is handled by IGMP
(Internet Group Management Protocol)
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Model of a Host IP
Implementation
Upper-Layer Protocol Modules
IP Service Interface
ICMP IGMP
IP
Module
Local Network Service Interface
Local Network Modules
(e.g Ethernet)
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ARP
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Extension to IP Service
Interface
• Basic functionality:
• Multicast IP datagrams are sent with specified IP host
group address rather than an individual IP address.
• Desired functionalities:
• Provides a way for upper layer to specify IP time-to•
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live of an outgoing multicast datagram
Provides a way for upper layer to identify which
network interface is used for multicast transmission
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Extension to Ethernet Local
Network Module
• Maps IP host group address to Ethernet
multicast addresses
• Low-order 23-bits of IP address to low-order 23-bits
of Ethernet address
• It is possible that multiple host group addresses are
mapped to one Ethernet address since the number of
significant bits of IP host group address is 28
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Extension to Receiver
• Extension to IP service interface
• An upper-layer protocol must ask the IP module
to join that group
• It is permissible to join the same group on
more than interface
• It is also permissible for more than one upperlayer protocol to request membership in the
same group
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Reverse-Path Multicast
(Dense Mode)
•
•
Source broadcasts each packet on its local network
Each router that receives a packet performs a Reverse Path
Forwarding(RPF) check
• Leaf router checks if it knows of any group member on its
attached subnets by IGMP queries
• Either forward or generate a prune message
• Prune message are forwarded back toward source
router
Leaf router
source
Local
Network
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Protocol Independent
Multicast - Sparse Mode
(PIM-SM)
• RP: Rendezvous Point
• Each group has a RP
• RP discovery is done using a bootstrap protocol
• Robustness: When the primary RP goes down, bootstrap
protocol can select an alternate RP
• Receivers send explicit join message to RP
• Each source sends multicast data packets
encapsulated in unicast packets, to RP
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Example of PIM-SM
RP
Join
R2
R1
R3
R4
R5
Shared tree
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Comparison between sparse
mode and dense mode
• Advantage of sparse mode
• Sparse mode has better scalability in terms of routing
•
state
Sparse mode is more efficient with the explicit join
message than tunnel establishment
• Disadvantage of sparse mode
• RP can be a single point of failure
• RP can become a host spot for multicast traffic
• Non-optimality may exist in the multicast tree
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Multicast Backbone (Mbone)
• Mbone is a set of multicast-capable
•
•
hosts/networks connected by tunnels
Mbone has no central management
New sites can be connected anywhere (flat
topology)
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Example of MBone
Multicast-capable
Network1
R1
Multicast-capable
Internetwork
R2
Network2
10.0.0.1
Destination = 2.x
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Destination=10.0.01
Destination=2.x
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Existing Problems in IntraDomain Protocol
• Scalability
• Most of the routes had long prefixes, which meant that
•
very few hosts could be represented in each routing
table entry
How to apply route aggregation and hierarchical
routing to mulitcast
• Manageability
• The utilization of tunnel may be redundant
• Domain boundaries
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Inter-Domain Multicast
• MBGP: Multiprotocol Border Gateway Protocol
• Review of BGP:
• Autonomous System (AS) boundaries between Internet domain
• ASes are commonly managed by different organizations
• Entities in one AS are not trusted by entities in another AS
• BGP provides policy control among ASes by advertising a
complete path
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Example of BGP
128.96
AS4
AS2
AS1
AS5
192.4.3
AS6
AS3
AS7
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MBGP(cont)
• The basic of MBGP:
• Subsequent Address Family Identifier fields are used to specify
•
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the cases: unicast, multicast, unicast/multicast
Each router only needs to know the topology of its own
domain and paths to reach each of other domain
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Example of MBGP
Intra-domain
cloud
Intra-domain
cloud
Intra-domain
cloud
Multicast-capable
Border router
(running MBGP)
Intra-domain
cloud
Unicast-only
Border router
(running BGP)
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Summary





Introduction of Multicast
Host Extension to IP multicast
Two examples of Intra-Domain Protocol for
multicast – dense and sparse modes
Existing deployment: MBone
One example of Inter-Domain Protocol for
multicast
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